INTERTWINED HISTORIES OF GLACIAL LAKE HITCHCOCK AND GLACIAL LAKE WINOOSKI IN THE BROOKFIELD QUADRANGLE, NORTH-CENTRAL VERMONT

Detailed mapping, as part of the National Cooperative Geologic Mapping Program in north-central Vermont, has revealed surficial materials and landforms that help elucidate the connections and timing of processes occurring in both Glacial Lake Hitchcock and Glacial Lake Winooski during and shortly after ice sheet retreat across the area. Using LiDAR shaded-relief imagery and modern mapping techniques the Brookfield, Vermont 7.5’ Quadrangle was mapped during the summer of 2021 and these results integrated with earlier mapping.

Physiographically, this region is dominated by a single north-south valley that hosts the drainage divide between the Winooski River basin to the north and the White River basin to the south. During ice sheet retreat a subglacial tunnel funneled meltwater southward along this valley evidenced by a segmented esker occurring along its length. A narrow, relatively shallow arm of Glacial Lake Hitchcock grew northward as the ice sheet retreated towards the drainage divide. Glacial Lake Winooski first formed when the ice sheet retreated north of the divide, subsequently growing and funneling all meltwater from the rapidly retreating/thinning ice sheet as well as meteoric water in the Winooski and Lamoille River basins through this outlet.

Small terraces, mapped south of the drainage divide, preserve parts of the delta built into Glacial Lake Hitchcock by the outflow from Glacial Lake Winooski. Immediately south of these terraces, large, ovoid lenses (bars) of coarse gravel are disconformably deposited on lake-bottom silt/clay. The gravel in these bars was likely sourced from the erosion of the delta and deposited in a valley-wide braided river subsequent to the drainage (or lowering) of Glacial Lake Hitchcock. The modern stream lacks the competence to move sediment of this size. Volume calculations indicate that only 10% of the presumed delta is preserved in the bars; finer sediments were carried farther down the valley.

Based on varve correlations, Glacial Lake Winooski existed between ~14,200 and ~13,800 y BP. During this time span (1) outflow from this lake built a substantial delta in Glacial Lake Hitchcock, (2) Glacial Lake Hitchcock drained from the Second Branch valley, and (3) that delta was largely eroded and redeposited as bars across the drained lake floor.